1. Field of the Present Invention
The present process is designed to yield a finely divided powdery composition by heating a particulate, sol-gel derived metal oxide composition containing one or more agents which generate gas upon heating to deagglomerate the particulate composition.
2. Description of the Prior Art
The formation of particulate, sol-gel derived metal oxide compositions containing small particles (e.g., of submicron size) having a tendency to generally agglomerate, particularly when isolated from solution, is well known. Such particulate compositions are formed by precipitation of the metal oxide or oxides by hydrolysis from liquid solutions so that a sol is first formed followed by collecting the precipitate which is then allowed to dry, as broadly indicated in U.S. Pat. No. 4,543,341 to E. A. Barringer et al. at Col. 1, line 64 to Col. 2, line 4. In such particulate compositions, the size of the primary particles is controlled by a number of factors relating to their method of synthesis, including, the rate of hydrolysis and rate of stirring, in the hydrated medium where the synthesis occurs. As mentioned in the Barringer et al. patent, the generally small particles formed by this type of method tend to agglomerate thereby causing problems in their later sintering to form sintered ceramic parts unless sintering additives are utilized.
Certain attempts have been made to avoid such agglomeration problems by controlling the preparative process in which the sol-gel derived metal oxides are formed.
For example, Barringer et al., in the aforesaid U.S. Pat. No. 4,543,341 and in Communications of the American Ceramic Society (December 1982), pages C-199-C-201, indicates that the stability against coagulation or agglomeration in aqueous dispersions of oxide powders requires a low electrolyte concentration and a solution pH that is several pH units above or below the isoelectric point of the oxide.
L. L. Hench and co-workers in Mat. Res. Soc. Symp. Proc., Vol. 32, pages 47-52 and 71-77, disclose the use of a drying control chemical additive, i.e. formamide, to prevent fracture upon drying of a sol-gel derived sodium oxide/silicon dioxide system. The drying control chemical additive was indicated as acting by reason of intramolecular bonding forces, i.e. due to hydrogen bond formation between the formamide amine groups and the hydroxide groups on the silica particle surfaces.